Rheostat



Feb. 11, 1964 c. B. HO'RSLEY 3,121,209

I RHEOSTAT Filed Nov. 24, 1961 2 Sheets-Sheet 1 Zak gal. 76

Nl/EN TOR CAP TON RSLEY 35 BY j A T TORNE V Feb. 11, 1964 c. B. HORSLEY 3,121,209

RHEOSTAT Filed Nov. 24, 1961 2 Sheets-Sheet 2 67 IN VE N TOR CAPE RTON B.H0RSLEY {6% M444. J! ATTORNEY United States Patent gluon Carbide Corporation, a corporation of New ork Filed Nov. 24, 1961, Scr. No. 154,604

9 Claims. (Cl. 338-193) The present invention relates generally to rheostats and, more particularly, to rheostats which employ carbon piles.

Hereto-fore, rheostats employing carbon piles have displayed a number of advantages over other types of variable electrical resistances. For example, carbon-pile rheostats can be subjected to higher overloads than most wire-wound rheostats Without burning out. Also, the resistance of a carbon pile can be varied in a continuous manner; although a continuously variable resistance is also obtainable with various water rheostats and slidewire resistances, such devices are relatively complicated mechanically and are usually impractical where rapid changes in resistance are required. However, although carbonpile rheostats have many advantages over other types of variable resistors, most of the carbon piles currently available have relatively narrow resistance ranges and relatively low power dissipating capacities.

It is, therefore, the main object of the present invention to provide a carbon-pile rheostat having a relatively broad resistance range.

It is another object of the invention to provide a carbonpile rheostat having a relatively high power dissipating capacity.

Other aims and advantages of the invention will be apparent from the following description and appended claims.

In the drawings: I FIG. ll is an elevation view of the front of a preferred embodiment of the inventive carbon pile rheostat;

FIG. 2 is an elevation view, partially in section, of the side of the apparatus shown in FIG. 1;

FIG. 3 is a slightly enlarged top View of the apparatus of FIG. 1;

FIG. 4 is an enlarged isometric view of one of the annular carbon elements and one of the electrodes used in the apparatus of FIG. 1;

FIG. 5 is a plan from line 5'5,in FlG. 2;

FiG. 6 is an elevation view from line '6-6 in FIG. 5;

FIG. 7 is an elevation view from line 7-7 in FIG. '6; and

FIG. 8 is an elevation view of one of the spring mountings from line 88 in FIG. 7.

in accordance with the present invention, there is proso as to vary the electrical resistancecf the.ca-rbon pile;

and at least two spaced apart electrodes in electrical con tact with the carbonpile. When the square of the thicle,

ness of each carbon element is less than 0.091 times the surface area of each face thereof, the carbon pile has a relatively broad resistance range. When the thickness of each carbon element is less than 611 times the minimum surface dimension of each face thereof, the carbon pile has a relatively large power dissipating capacity. Thus, the dimensions of the carbon elements can'be chosen to satisfy one or both of the aforedescr-ibed'conditions,-de

Patented Feb. 1 l 1964 pending on the operating characteristics required in the rheostat.

Heretofore, conventional carbon piles for use as rheostats have been made of relatively rigid carbon discs, e.g., /2 inch in diameter by inch thick, 1 inch in diameter by /8 inch thick, 2 inches in diameter by inch .thick. The larger discs are generally employed where a low resistance and high power dissipating capacity are desired, While the smaller discs are used Where a high resistance and low power dissipating capacity are desired. In most such conventional carbon-pile rheostats, the resistance can be varied only by a factor of about eight or less. Also, it has been found that the resistance of a typical carbonpile rheos-tat decreases approximately in inverse proportion to the applied pressure only up to a compressive force of about ten pounds, and that further increases in pressure result in relatively small changes in resistance, regardless f the length or diameter of the carbon pile.

In the rheostat of the present invention, the individual carbon elements are relatively thin and flexible and, therefore, provide a greater number of face-to-face contact points as the applied pressure is increased. As a result, the resistance range of the novel rheostat is significantly increased, and the resistance decreases approximately in inverse proportion to the applied pressure up to compressive forces as high as pounds. The maximum resistance of the inventive rheostat is determined mainly by the number of elements or face-to-face contacts in the carbon pile. The maximum power dissipating capacity of the carbon pile is determined by its critical temperature, i.e., the temperature at which the surfaces of the carbon elements become non-uniformly heated and hot spots begin to develop. if the temperature is increased above the critical temperature, the carbon elements may become cracked or pitted, and portions thereof may be converted into carbon dioxide. Also, the rheostat performs erratically because its resistance characteristic becomes un stable. Thus, the power dissipating of a carbon pile is generally limited to a value which does not raise its temperature above the critical temperature. In the case of the inventive carbon pile, the novel proportions of the carbon elements increases the critical temperature of the .pile and thus increase the maximum power dissipating capacity of the pile.

A preferred embodiment of the inventive rheostat will now be described in detail by referring to the drawings.

With reference to the drawings, the novel rheostatcomprises, in general, a base 15, an upright housing 11' mounted on the base 15, a spring-supported carbon pile 10 mounted within the housing .11, and a solenoid 22 ontop of the housing 11 for applying a variable compressive force to the carbon pile 1o. The supporting base 15 is bolted to flanges -14 on the lower end of the housing 11. As shown in FIG. 2, the base 15 includes a. raised central portion 16 on which rests a vertically disposed guide rod 17. The guide rod 17 is rigidly fastened to the base 15 by means of a machine bolt 1-8 and extends upwardly through the center of the carbon pile 1t} and the housing 11. The top of the guide rod 17 is provided with a locating pin 25 which is press-fitted into a central bore in member 19 (see FIG. 6).

Referring to FIG. 3, it can be seen that the housing 11 in section is generally cylindrical, but has flat face portions '12 extending in parallel planes along opposite sides thereof. The face portions 12 are provided with vertically spaced slots 13 to permit electrical connections-to the carbon pile, as hereinafter described. As'shown inFIG.

6, members 19 and 21 are fastened to the top of the housing 11 and support a linear actuator or solenoid 22. The solenoid shank 23 extends downwardly through a hole in the center of member 21 and is rigidly attached to a spider 24. The spider 24 has four arms extending radially from its axis, and each arm is provided with a vertical bore which tightly retains a pressure rod 26 so that the spider 24 and rods 26 form a rigid assembly. To fix the position of the spider 24 and rods 26 horizontally, member 19 is provided with circumferentially spaced clearance holes; the rods 26 extend downwardly through the clearance holes into contact with a ring 27 of insulating material. The ring 27 is held stationary in an axial direction by means of four ribs 28, which are also formed of insulating material and which are retained in longitudinal slots in the surface of guide rod 17. Thus, it can be seen that the positions of the rod 17 and member 19 are fixed, while the assembly of spider 24 and rods 26 is free to vary slightly in a vertical direction in accordance with the force exerted by the solenoid shank 23.

Returning to FIG. 2, it can be seen that an insulating ring 31 supported by the base 15 is in face-to-face contact with a graphite electrode 33. Similarly, at the top of the housing 11 (see FIG. 6), a graphite electrode 32 is in face-to-face contact with the insulating ring 27. A carbon pile of thin annular carbon elements 34 is disposed between electrodes 33 and 32 and within the housing 11, the top and bottom elements 34 of the pile being in faceto-face contact with electrodes 32 and 33, respectively. The outer diameter of the individual carbon elements 34 is slightly smaller than the inner diameter of the housing 11 so as to leave an annular space between the pile and the housing. Intermediate graphite electrodes 37 are dis- ]posed between groups of the carbon elements 34 at uniform intervals, as illustrated in FIG. 2. In the embodiments shown in the drawings, there are 162 annular carbon elements divided into nine groups of eighteen by the intermediate electrodes 37. Although most conventional carbon pile rheostats employ metal electrodes, the preferred electrode material in the present invention is graphite. Also, it is preferred to apply the compressive force to the carbon pile through the electrodes. The use of graphite electrodes both at the ends of the carbon pile and within the pile eliminates the high resistance region which is often present between the electrodes and the carbon elements in contact therewith when metal electrodes are used. For example a pressure change from 1 p.s.i. to p.s.i. between two contacting surfaces reduces the electrical resistance by a factor of two if the surfaces are silver and carbon, or by a factor of three if the surfaces are aluminum and carbon, whereas the same pres sure change reduces the resistance by a factor of ten if the contacting surfaces are graphite and carbon. Also, a carbon-to-metal contact may lead to fluctuating operating characteristics as a result of surface oxidation or burning at hot spots.

Both the carbon elements and the graphite electrodes employed in the preferred embodiment of the present invention are shown more clearly in FIG. 4. Each carbon element 34 is a flat ring having dimensions such that the square of the thickness of each element is less than 0.001 times the surface area of each face thereof and/or the thickness of each element is less than 0.1 times the minimum surface dimension of each face thereof. Although it is usually preferred to have the dimensions of the carbon elements meet both of the aforedescribed conditions, only the first condition may be met if only a broad resistance range is desired, and only the second condition may be met if only a high power dissipating capacity is desired.

As can be seen in FIG. 4, the inner and outer diameters of the annular portion of the graphite electrodes are about the same as the inner and outer diameters of the carbon elements. Also, the graphite electrodes are provided with tabs 35 with bores 36 therein for making electrical connections to the electrodes. Referring to FIG. 6, the tabs 35 extend out through the slots 13 in the faces 12 of the housing 11 and between a plurality of cover pieces 41 bolted to the housing 11. The cover pieces 41 overlap the slots 13 sufliciently to permit only a small amount of vertical movement of the tabs 35. Afiixed to each adjacent pair of cover pieces 41 is a bracket 42 which has a shelf portion to support a pair of Vertically extending compression springs 43. The lower ends of the compression springs 43 are retained in cups 44 which are fastened to the shelf portion of bracket 42, while the upper ends of the springs 43 are retained in cups 45 which are riveted to a yoke 46. The yoke 46 overlies one of the electrode tabs 35 and is bored in the middle to receive a bolt 47. The tabs and yokes are fastened together by means of the bolt 47 and a nut 48. Thus, the yokes actually carry the electrodes, which are urged upwardly by the springs 43 to counteract the weight of the carbon elements resting thereon. As illustrated in FIGS. 1 and 2, a similar spring-supporting assembly is provided for each protruding electrode tab 35. The spring supports permit vertical stacking of the carbon rings without producing a higher pressure on the lower rings in the stack, thereby avoiding any significant reduction in the resistance range of the carbon pile. In other words, the upward force exerted by the springs on the graphite electrodes substantially eliminates gradients in the compressive pressure throughout the carbon pile. Of course, the upward force of the springs must be sufiiciently small to maintain electrical contact between the graphite electrodes and the carbon elements below the electrodes. It is obvious that an upward force similar to that provided by the spring mountings described above could be provided by other means, such as a hydraulic supporting system.

Referring to FIG. 7, flexible copper strips 51 are fastened to certain of the yokes 46 to serve as means for making external electrical connections to the graphite electrodes. The copper strip 51 extends outwardly from beneath the head of the bolt 47 for a short distance, and then downwardly for connection to a terminal stud 52. The assembly for connecting the electrodes to the terminal stud 52 is shown most clearly in FIG. 3. Vertically extending flanges 53 are formed integrally with the housing 11 adjacent the flat faces 12, and a bus bar 54 is rigidly fastened to the flanges 53 by means of standoff insulators 55 and bolts 56. The bolts 56 are threaded into the flanges 53 and are provided with insulating sleeves 57 so that no electrical contact is made between the bolts 56 and the bus bar 54. Also, there is an insulating cap or washer 59 between the bolthead and the bus bar 54. The terminal stud 52 is fastened to the center of the bus bar 54, and a connecting strip 51 is secured to each end of the bus bar 54. Connecting strips 51 are employed at both ends of the graphite electrode to ensure a uniform distribution of current and to eliminate any tendency for the electrode to cant under operating conditions. As shown in FIGS. 1 and 2, electrical connections to other electrodes in the carbon pile are made in a similar manner, except that some of the connecting strips 51 extend downwardly and some upwardly. The exact arrangement of the connections depends mainly on the proposed use for the rheostat; the arrangement shown in the drawings is such that the groups of carbon elements can be connected in a delta configuration. If a single two-terminal rheostat is all that is required, only two flexible connections would be required at each end of the housing, i.e., one pair for electrode 32 and another pair for electrode 33.

As mentioned above, the resistance of the carbon pile is varied by varying the compressive force applied thereto. Although the compressive force may be applied by any suitable means, it is preferred to employ a solenoid, such as that shown in FIG. 6. The solenoid 22 is mounted on member 22, and the compressive force is transmitted from the solenoid shank 23 (see FIG. 6) to the carbon pile through spider 24, pressure rods 26, insulating ring 27, and electrode 32. The solenoid coil 60 is enclosed in insulation 62 and mounted on a bottom plate 64 within a housing 66. The housing 66 is secured to the bottom plate 64 by screws. Resting on the solenoid coil 60 and within the housing 66 is a bushing 68. The housing 66, the bottom plate 64, and the bushing 68 are all part of the magnetic circuit and, therefore, are all made of magnetic material. The bottom plate 64 is pro vided with a central bore "ill with a large upper counterbore 72. Seated in the counterbore 72 and attached thereto by screws is a washer '74- of magnetic material with a central bore of about the same diameter as bore 76.

The heavy solenoid core 76 of magnetic material is slidably mounted within a non-magnetic brass cylindrical sleeve 78, which in turn is mounted within the solenoid coil 69 and silver-soldered to the washer 74. Projecting centrally from the bottom of the core 7 6 is a non-magnetic aluminum shank 23, which extends through the washer 74- and the hole 76 and is rigidly attached to the spider 24. A non-magnetic washer 82 is disposed around the shank 23 between the core 76 and the washer 74. A dust cap 84 is fitted over the top of the entire solenoid assembly. The electrical leads $6 to the solenoid coil tl extend through a slot 88 in the bottom plage 6d and are connected to a suitable control circuit, which controls the pressure exerted by shank 23 in the spider 2 One eX- ample of a control circuit suitable for use with the solenoid described above is described in detail in US. Patent No. 2,813,181, entitled Automatic Current Controller. Since the control circuit forms no part of the present invention, a description thereof will not be repeated herein.

In order to illustrate the advantages of the present invention, three rheostats were prepared with three different sets of carbon rings. The operating characteristics of the three rheostats are given in the following table. The carbon pile in rheostat A was made of rings of 2.25- inch inside diameter, 4.75-inch outside diameter, and 0.0625-inch thickness, i.e., the square of the thickness of each element was less than 0.001 times the surface area of each face thereof, and the thickness of each element was less than 0.1 times the width thereof. The carbon pile in rheostat B was made of rings of 3.0-inch inside diameter, 4.0-inch outside diameter, and 0.0625-inch thickness, i.e., the square of the thickness of each element was less than 0.001 times the surface area of each face thereof, but the thickness of each element was more than 0.1 times the width thereof. The carbon pile in rheostat C was made of conventional rings of 2.25-inch inside diameter, 4.75-inch outside diameter, and 0.375-inch thickness, i.e., the dimensions of the elements did not conform to either of the conditions of the present invention.

Although the aforedescribed annular shape is preferred for the carbon elements in the inventive rheostat, it is to be understood that carbon piles can be built within the scope of the present invention without employing an annular configuration. For example, the carbon elements could be made in the form of triangles or quadrangles, with or without central openings. Also, the edges of the carbon elements may be irregular or provided with toothlike projections, although this does little to increase the power dissipating capacity of the carbon pile. The annular shape is preferred because it provides a more flexible carbon element for a given amount of carbon, and the contact points where hot spots develop tend to be more uniformly distributed over the surfaces. The annular configuration also provides both inner and outer edges for cooling purposes.

It should be further pointed out that the carbon elements need not be stacked vertically, although that is the preferred form. For example, the carbon elements could be stacked horizontally or on an incline. Furthermore, the compressive force can be applied by means other than a solenoid, such as by a simple adjustable screw or other means well known in the art.

While various specific forms of the present invention have been illustrated and described herein, it is not intended to limit the invention to any of the details herein shown, but only as set forth in the appended claims.

What is claimed is:

1. A rheostat comprising a carbon pile of thin carbon elements in fazce-to-face contact, the square of the thickness of each of said elements being less than 0.001 times the surface area of each face thereof; means for applying a compressive force to said carbon pile in a direction sub stantially perpendicular to the faces of said carbon elements; means for varying said compressive force so as to vary the electrical resistance of said carbon pile; and at least two spaced apart electrodes in electrical contact with said carbon pile.

2. A rheostat as defined in claim 1 wherein the thicknessof each of said carbon elements is less than 0.1 times the minimum surface dimension of each face thereof.

3. A rhecstat comprising a carbon pile of thin carbon elements in face-to-face contact, the thickness of each of said elements being less than 0.1 times the minimum surface dimension of each face thereof; means for applying a compressive force to said carbon pile in a direction su b- Dimensions (inches) 2% ID. x 4% O.D. 3 ID. x 4 O.D. x 2% LD. 114% O.D.

X lie M6 11% No. Discs 18 3 Compressive Force (pounds). A 8 128 8 128 V 8 128 Resistance (ohms) 165 050 005 165 050 005 02G 008 003 Critical Temperature F.) 450 475 475 300 300 30-0 225 250 250 Max. Safe Temperature 425 450 450 275 275 275 200 225 225 Power Dissipation (wattsfi... 270 400 400 200 200 200 145 1 For continuous operation.

2 At maximum sale temperature and 0.1111. cooling.

It can be seen from the table that the same amount of carbon was present in rheostats A and C. However, with identical compressive forces, the resistance range of rheostat A varied by a factor of 33, while the resistance range of rheostat C varied by a factor of less than 9. In other words, the resistance range of the inventive rheostat (A) was more than three times as great as that of the conventional rheastat (C). Furthermore, the power dissipating capacity of the inventive rheostat (A) was more than twice as great as that of the conventional device (C). Rheostat B, which was designed to produce an increased resistance range only, had a resistance range about the same as that of rheostat A, but its power dissipating capacity was closer to that of the conventional rheostat (C).

stantially perpendicular to the faces of said carbon elements; means for varying said compressive force so as to vary the electrical resistance of said carbon pile; and at least two spaced apart electrodes in electrical contact with said carbon pile.

4. A rheostat comprising a carbon pile of thin carbon elements in face to-face contact, the square of the thickness of each of said elements being less than 0.001 times the'surface area of each face thereof; a first graphite electrade in contact with a first end of said carbon pile and a second graphite electrode in contact with a second end of said carbon pile; means. for applying :a compressive force to said carbon pile through said graphite electrodes in a direction substantially perpendicular to the faces of said carbon elements; and means for varying said compressive force so as to vary the electrical resistance of said carbon pile.

, 5. A rheostat as defined in claim 4 wherein the thickness of each of said carbon elements is less than 0.1 times the minimum surface dimension of each face thereof.

6. A rheostat comprising a vertically stacked carbon pile of thin carbon elements in face-to-face contact, the square of the thickness of each of said elements being less than 0.001 times the surface area of each face thereof and the thickness of each of said elements being less than 0.1 times the minimum surface dimension of each face thereof; a first graphite electrode in contact with the upper end of said carbon pile and a second graphite electrode in contact with the lower end of said carbon pile; at least one intermediate graphite electrode disposed between and in contact with adjacent carbon elements be tween the ends of said carbon pile; means for applying a compressive pressure to said carbon pile through said graphite electrodes in a direction substantially perpendicular to the faces of said carbon elements; means for varying said compressive pressure so as to vary the electrical resistance of said carbon pile; and means for applying an upward force to said intermediate electrodes so as to substantially eliminate gradients in said compressive pressure throughout said carbon pile.

7. A rheostat as defined in claim 6 wherein the thickness of each of said carbon elements is less than 0.1 times the minimum surface dimension of each face thereof.

8. A rheostat comprising a vertically stacked carbon pile of thin, annular carbon elements in face-to-face contact, the square of the thickness of each of said elements being less than 0.001 times the surface area of each face thereof and the thickness of each of said elements being less than 0.1 times the minimum surface dimension of each face thereof; a first annular graphite electrode in face-to-face contact with the upper end of said carbon pile and a second annular graphite electrode in face-toface contact with the lower end of said carbon pile; a plurality of intermediate annular graphite electrodes disposed between and in contact with adjacent carbon elements at intervals between the ends of said carbon pile; means for applying a compressive pressure to said carbon pile through said graphite electrodes in a direction substantially perpendicular to the faces of said carbon elements; means for varying said compressive pressure so as to vary the electrical resistance of said carbon pile; and means for applying upward forces to said intermediate electrodes so as to reduce the pressure on the carbon elements below said intermediate electrodes While maintaining electrical contact between said intermediate electrodes and the carbon elements below said electrodes.

9. A rheostat as defined in claim 8 wherein the thick ness of each of said carbon elements is less than 0.1 times the minimum surface dimensions of each face thereof.

References Cited in the file of this patent UNITED STATES PATENTS 774,460 Waddel Nov. 8, 1904 1,260,994 Bidur Mar. 26, 1918 1,984,947 Schweibold Dec. 18, 1934 2,398,679 Thompson Apr. 16, 1946 

1. A RHEOSTAT COMPRISING A CARBON PILE OF THIN CARBON ELEMENTS IN FACE-TO-FACE CONTACT, THE SQUARE OF THE THICKNESS OF EACH OF SAID ELEMENTS BEING LESS THAN 0.001 TIMES THE SURFACE AREA OF EACH FACE THEREOF; MEANS FOR APPLYING A COMPRESSIVE FORCE TO SAID CARBON PILE IN A DIRECTION SUBSTANTIALLY PERPENDICULAR TO THE FACES OF SAID CARBON ELE- 